3D window renders, but Image Render doesn't?
Okay, I figured out what I was doing wrong.  To reduce the clutter in the scene I had turned off visibility of some of the panels (red below). BUT...I did NOT turn off rendering (blue). Inadvertently I had backed the camera behind an invisible plane that had no illumination. When I turned off rendering for the hidden plane things worked fine. So in the preview rendering of the 3d window, what you see is what you get.  But in the full render, what you don't see can get you too!